P. Spanne

Medial axis analysis of void structure in three-dimensional tomographic images of porous media

We introduce the medial axis as a tool in the analysis of geometric structure of void space in porous media. The medial axis traces the fundamental geometry of the void pathways. We describe an algorithm for generating the medial axis of the void structure from digitized three dimensional images of porous media obtained from X ray CAT scans. The medial axis is constructed during an iterative erosion procedure which, at each step, replaces the image of the void structure with a smaller version obtained by eroding its surface layer of voxels. The algorithm is applied to high (5 μm) resolution microtomographic images of two rock chips (Berea sandstone and Danish chalk) and a sample of uniform (100 μm) diameter, packed glass beads. We statistically investigate several geometrical properties of the structure of the medial axes obtained. The first is the distribution of relative volumes in each erosion layer of the void space. We find the distributions to be exponential for the two real rock samples and normal for the packed glass beads. The second property investigated is the distribution of volumes of disconnected segments of the medial axis which are in one-to-one correspondence with disconnected void segments of the sample. We find indications for a universal power law behavior governing the distribution of volumes of the smallest disconnected pieces. The final behavior studied is a geometric tortuosity as measured by shortest paths through the medial axis. This tortuosity distribution appears well described by a gamma distribution.

Neuropathology of ablation of rat gliosarcomas and contiguous brain tissues using a microplanar beam of synchrotron-wiggler-generated X rays

Adult‐rat‐brain tissues display an unusually high resistance to necrosis when serially irradiated with parallel, thin slices of a microplanar (i.e., microscopically thin and macroscopically broad) beam of synchrotron‐wiggler‐generated, approx. 35–120 keV (median approx. 50 keV) Gd‐filtered X rays at skin‐entrance absorbed doses of 312 to 5000 Gy per slice. Such microplanar beams were used to irradiate young adult rats bearing right frontocerebral 9L gliosarcomas (approx. 4 mm diameter), through a volume of tissue containing the tumor and contiguous brain tissue, either in a single array or in 2 orthogonally crossed arrays of tissue slices. Each array included 101 parallel microplanar slices, 100 μm center‐to‐center distance, each slice being approx. 25 μm wide and 12 mm high, with skin‐entrance absorbed doses of 312.5 Gy or 625 Gy per slice. Compared with unirradiated controls with a median survival time of 20 days after tumor initiation, the median survival time was extended in irradiated rats by 139 days (625 Gy, crossed arrays), 96 days (312.Gy, crossed arrays) or 24 days (625 Gy, single array). The tumors disappeared in 22 of the 36 irradiated rats, 4/ 11 even after unidirectional microbeam irradiation. The extent and severity of radiation damage to the normal brain in rats with or without tumor was graded histopathologically. Correlation of those grades with radiation doses shows that loss of tissue structure was confined to beam‐crossing regions and that only minor damage was done to zones of the brain irradiated unidirectionally. Int. J. Cancer 78:654–660, 1998.

Computerized microtomography using synchrotron radiation from the NSLS

Results of microtomography experiments that employ filtered radiation from the National Synchrotron Light Source X-26 Microprobe beam line are presented. These experiments have yielded images of a freeze-dried caterpillar with a spatial resolution of the order of 30 μm and show that the limit on the spatial resolution with the present apparatus will be 1 to 10 μm. Directions for improvement in synchrotron microtomography techniques and some possible applications are discussed.

Design of a multislit, variable width collimator for microplanar beam radiotherapy

Microbeam radiation therapy of the intracerebral 9L gliosarcoma in rats, an experimental surrogate for human malignant gliomas, using mainly 30–130 keV wiggler‐generated x rays, extended the residual lifespans of some rats ten or more times over those of untreated, similar gliosarcoma‐bearing rats. The rats were exposed 300 or 600 times to an upright, 25‐μm‐wide, 4‐mm‐high x‐ray beam. A multislit collimator has been designed to shorten the time required for the therapy.

Catalyst analysis using synchrotron X-ray microscopy

Abstract Synchrotron X-ray microscopy techniques have been used to characterize several different types of heterogeneous catalysts. Using a collimator to produce microbeams with a size of less than 10μm, maps of the elemental distributions based on detection of the fluorescent X-rays were made, and computed microtomography (CMT) techniques were applied to produce “ phase/density contrast” maps. CMT techniques were also used to determine the distribution of a specific major element by making measurements above and below the K X-ray absorption edge. The measurements were made using bending magnet radiation and, in one case, radiation from a 4 T superconducting wiggler at the National Synchrotron Light Source. Examples of applications to the study of polyethylene polymerization particles, fluid catalytic cracking catalysts, and hydrotreating catalysts are given.

Determination of polymerization particle morphology using synchrotron computed microtomography

Abstract Polymerization of monomers over heterogeneous catalysts results in the fragmentation of the catalysts and subsequent transport in the polymer particles that are produced. Characterization of the process using nondestructive synchrotron computed microtomography techniques makes possible measurement of the distribution of the catalyst fragments in an individual particle and, in addition, gives an estimate of the particle porosity and surface area. The present experiment was carried out using the X-ray microscopy facility at the Brookhaven National Synchrotron Light Source (NSLS) X26 beam line. The tomographic sections were analyzed using autocorrelation techniques to determine porosity and surface area values. The results are compared to values obtained using conventional methods. This procedure makes possible the extraction of quantitative information about porosity and specific area from the tomograms.

Laue diffraction protein crystallography at the National Synchrotron Light Source

A new facility for the study of protein crystal structure using Laue diffraction has been established at the X26 beam line of the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory. The characteristics of the beam line and diffraction apparatus are described. Selected results of some of the initial experiments are discussed briefly by beam line users to illustrate the scope of the experimental program. Because the Laue method permits the recording of large data sets in a single shot, one goal in establishing this facility has been to develop the means to study time-resolved structures within protein crystals. Systems being studied include: the reactions catalyzed by trypsin; photolysis of carbonmonoxy myoglobin; and the photocycle of photoactive yellow protein.

Cell survival after Auger electron emission from stable intracellular indium exposed to monochromatic synchrotron radiation

The biological effect of Auger electrons emitted from indium in V79 cells was investigated. K-shell vacancies were induced by synchrotron x-rays. Two energies, 100 eV above and below the K-edge of indium, were used. The cell survival for controls was similar to that which has been reported by others, with D37 = 4.4 Gy. Indium-oxine-labelled cells exhibited a survival clearly below that of the controls, D37 = 3.2 Gy, but no significant difference in survival between irradiations above and below the K-edge could be observed. The explanation is, inter alia, that the number of photons interacting with indium atoms incorporated into the cell, is small compared with the number of photons interacting with other atoms in the cell. The toxicity of indium oxine made it impossible to incorporate a sufficient number of indium atoms into the cells to observe a difference in this study. However, monoenergetic irradiation above and below the K-edge, provides a technique for the investigation of basic biological effects of Auger processes.

New Insight into The Changing Catalyst/Polymer Morphology During Olefin Polymerization: The Application of Tomography

Abstract Heterogeneous olefin polymerization increasingly dominates the production of polyethylene, polypropylene and mixed polyolefins. Our studies of the morphology of these catalysts reveal that several different mechanisms occur for the changes in the accessible active surface, the void volume and the distribution of catalytically active particles within the growing polymer. These differences in morphological dynamics are dictated by the initial catalyst pore structure and the related subsequent catalyst fragmentation processes. MgCl 2 supported catalyst tend to be uniform in their microporosity (i.e., ∼ 4 nm); although, there is evidence for multi-modal μ-pore dimension due to the agglomeration of small subparticles. The initial fragmentation proceeds readily and uniformly to yield a multi-grain growth of sub-particle agglomerates. The surface of these sub-particle agglomerates is accessible through the void-space between growing catalyst/particle grains. Silica supported catalysts exhibit a network of porosity from meso- to micro-pores. These catalysts fragment progressively from the larger to the smaller meso-pores following the filling of the available pore space with growing polymer. Since this fragmentation is progressive and is dictated by pore filling, the surface area and pore volume can decrease rapidly. In the extreme, these mechanisms result in both loss and retention of available surface area and pore volume. As the polymer grows these differences can result in uniform as well as non-uniform distributions of catalyst fragments within the growing polymer. However, the differences between the morphological mechanisms can depend on the conditions (pressure, temperature and gas phase v.s. slurry) of the polymerization process and, thus, no simple conclusions may be drawn that relate one catalyst system to a specific mechanism for the changes in morphology.

Electron microscopy and computed microtomography studies of in vivo implanted mini-TL dosimeters

The need for direct methods of measuring the absorbed dose in vivo increases for systemic radiation therapy, and in more sophisticated methodologies developed for radioimmunotherapy. One method suggested is the use of mini-thermoluminescent dosimeters (TLD). Recent reports indicate a marked loss of signal when the dosimeters are used in vivo. We investigated the exterior surface of the dosimeters with scanning electron microscopy and the interior dosimeter volume with computed microtomography. The results show that the dosimeters initially have crystals uniformly embedded in the teflon matrix, with some of them directly exposed to the environment. After incubation in gel, holes appear in the dosimeter matrix where the crystals should have been. The computed microtomographic images show that crystals remain in the interior of the matrix, producing the remaining signal. We conclude that these dosimeters should be very carefully handled, and for practical use of mini-TLDs in vivo the dosimeters should be calibrated in equivalent milieus. An alternative solution to the problem of decreased TL efficiency, would be to coat the dosimeters with a thin layer, of Teflon, or other suitable material.